The present invention relates to improved methods of coating the outer and inner exposed surfaces of the seam of a welded metal tube with a protective metal coating. The tube may be formed from a coated metal strip such as zinc coated steel. However, the coating on the surfaces of the seam will burn off or vaporize during welding because the melting temperature of the coating, such as zinc or aluminum and their alloys, has a much lower melting temperature than the tube which is generally formed of steel. More specifically, the improved method of this invention utilizes a unique heating and coating process which results in an improved bond between the protective metal coating and the exposed seam surface which extends the life of the tube.
Methods of continuous or in-line forming of seamed metal tubes such as steel from a continuous strip are well known. In a conventional tube forming mill, the strip is rolled to form an open seam tube having nearly abutting edges located at the top of the tube. The edges are then welded together by one of several conventional methods which generally include heating the edges and then either forging the edges together with squeeze rolls and/or flux welding a seam. The edges of the tube may be heated, for example, by resistance welding, electric arc or by high frequency induction welding. High frequency induction welding is a form of electronic resistance welding, wherein the open seam tube is received through an electric work coil which creates a strong magnetic field, which in turn induces a current to flow around the tube and in the "Vee" formed as the edges of the strip are welded. An impeder is generally located within the tube, which forces the current down the nearly abutting edges of the open seam tube, heating the tube edges to hot forging temperature. The tube edges are then forged by squeeze rolls which drive the molten edges of the open seam tube together to form an integral seam.
In-line galvanizing and coating or painting processes which apply a protective coating to the tube are also well known. The strip may be galvanized or painted on one or both sides prior to forming and welding, or the outer surface of the welded seamed tube may be galvanized by immersing the tube in a molten zinc bath. Where the strip is coated with a protective coating prior to seam welding, the coating on the seam area will burn-off or vaporize because the welding operation involves the melting of the tube material, which is generally steel. The temperature of the adjacent edges of the open seam tube are heated to melting temperature of steel, which is 2300.degree. F. or greater. Where the strip is coated with protective metal coating, such as zinc or aluminum, the protective metal coating will melt and vaporize during welding and the protective metal coating near the weld will flow away from the seam, which is located at the top of the tube in a conventional tube mill. Zinc coating solutions have also been used to "paint" the exterior surface of the seam. However, such coatings have poor adherence and are mainly cosmetic. "Metallizing" of the seam surface has also been attempted; however, such coatings are mainly mechanical and do not provide a metallurgical bond between the protective metal coating and the steel tube. The failure of the prior commercial processes to fully coat and thus protect the tube seam is evident by the fact that the welded area is generally the first to fail in accelerated corrosion tests. Metallized surfaces covering the exterior seam surfaces have been known to fail because of mechanical stresses associated with tube fabrication.
The improved method of coating the seam surfaces of a welded tube of this invention assures complete coating of the outer and inner surface of the seam of a welded metal tube and a metallurgical bond between the protective metal coating on the exterior surface of the weld in a continuation or in-line process utilizing conventional tube forming processes and coating equipment.